(1) Field of the Invention
The present invention relates to a high-pressure discharge lamp, and particularly to a technique for hermetically sealing tube end parts of an arc tube that is made from a translucent ceramic material.
(2) Related Art
As a typical material for an arc tube used in a metal halide lamp that is one type of a high-pressure discharge lamp, silica glass has been conventionally used. In recent years, however, an arc tube made from a translucent ceramic material has been developed and commercialized. Because translucent ceramic has a higher heat resistance than silica glass, a metal halide lamp using a translucent ceramic arc tube can be lit at higher temperatures and can exhibit better lamp characteristics such as color rendering properties than a metal halide lamp using a silica glass arc tube.
In the commercialization process, however, such a translucent ceramic arc tube was found to require a relatively long total length for the following reason. A frit-sealing technique is employed to seal a translucent ceramic arc tube. Here, ceramic cement (frit) is used as a sealing material. At high temperatures, such a frit reacts with a metal halide that is a light-emitting material used in the arc tube. To prevent this reaction from occurring, parts (tube end parts) to be sealed using the frit need to be positioned away from a high-temperature part (a discharge space).
The resulting long arc tube inevitably degrades the compactness of a metal halide lamp as a whole. Further, the heat capacity of such long arc tube as a whole is high, thereby degrading the luminous efficiency and failing to satisfy the recent demands for energy-saving.
In view of this, a technique for sealing by way of metallizing (hereafter referred to as a xe2x80x9cmetallize-sealing techniquexe2x80x9d) as disclosed in Japanese Laid-open Patent Application Nos. 2000-100385 and 2001-58882 is now calling attentions as a new sealing technique. A sealing part formed according to the metallize-sealing technique has been known to be less reactive to a metal halide and to provide stronger sealing than a sealing part formed according to the above frit-sealing technique. Particular techniques disclosed in the above-cited applications further enable thermal shock resistance to be improved by providing an impregnated glass phase in each sealing part formed according to the metallize-sealing technique.
However, these disclosed techniques are found to have various problems. An excessively shortened arc tube with both sealing parts being too close to a high-temperature part may suffer from such a problem that its inner surface is blackened and thereby the luminous flux is greatly degraded. The excessively shortened arc tube with both sealing parts being too close to a high-temperature part may also suffer from such problems that its sealing parts are cracked, and that luminescent colors are changed due to a material for the impregnated glass phase being eroded by a metal halide that is a light-emitting material used in the arc tube.
The first object of the present invention is to provide a high-pressure discharge lamp that can use an arc tube whose total length is as short as possible and that can prevent such a problem as blackening of the arc tube. The second object of the present invention is to provide a high-pressure discharge lamp that can use an arc tube whose total length is as short as possible and that can prevent such problems as crack generation and luminous color change.
The first object of the present invention can be achieved by a high-pressure discharge lamp, including: an arc tube that is made up of a main-tube part in which a discharge space is formed, and two thin-tube parts extending from both ends of the main-tube part, the main-tube part and the two thin-tube parts being made from a translucent ceramic material; and a pair of electrodes having rods that respectively extend through the two thin-tube parts into the discharge space so that tops thereof face each other with a predetermined distance in-between, the rod of at least one of the electrodes being held by a tubular electrode holder embedded in and bonded to the thin-tube part via an adhesive agent, the electrode-holder being made of a halide-resistant metal, the adhesive agent including a sintered halide-resistant metal impregnated with mixture glass, wherein the electrode holder is at such a position that satisfies the expression xe2x80x9cLxe2x89xa70.012P+2.5 [mm]xe2x80x9d where xe2x80x9cLxe2x80x9d is a distance [mm] between (a) a top of the electrode whose rod is held by the electrode holder and (b) one end of the electrode holder closer to the discharge space, and xe2x80x9cPxe2x80x9d is a lamp wattage [W].
According to this construction, a glow discharge is not generated from the end of the conductive electrode holder at the discharge space side when the lamp is started. Therefore, the blackening phenomenon of the inner surface of the arc tube can be prevented during the effective lifetime of the lamp. Further, the thin-tube part can be shortened in a range of the distance xe2x80x9cLxe2x80x9d calculated using the above expression, so that the luminous efficiency can be improved as compared with a conventional lamp employing the frit-sealing technique.
The second object of the present invention can be achieved by a high-pressure discharge lamp, including: an arc tube that is made up of a main-tube part in which a discharge space is formed, and two thin-tube parts extending from both ends of the main-tube part, the main-tube part and the two thin-tube parts being made from a translucent ceramic material; and a pair of electrodes having rods that respectively extend through the two thin-tube parts into the discharge space so that tops thereof face each other with a predetermined distance in-between, the rod of at least one of the electrodes being held by a tubular electrode holder embedded in and bonded to the thin-tube part via an adhesive agent, the electrode holder being made of a halide-resistant metal, the adhesive agent including a sintered halide-resistant metal impregnated with mixture glass, wherein a temperature of one end, closer to the discharge space, of a bonding area formed using the adhesive agent does not exceed a lowest temperature at which an erosion action of a light-emitting material enclosed in the discharge space on the mixture glass occurs. The second object of the present invention can also be achieved by a high-pressure discharge lamp, including: an arc tube that is made up of a main-tube part in which a discharge space is formed, and two thin-tube parts extending from both ends of the main-tube part, the main-tube part and the two thin-tube parts being made from a translucent ceramic material; and a pair of electrodes having rods that respectively extend through the two thin-tube parts into the discharge space so that tops thereof face each other with a predetermined distance in-between, the rod of at least one of the electrodes being held by a tubular electrode holder embedded in and bonded to the thin-tube part via an adhesive agent, the electrode holder being made of a halide-resistant metal, the adhesive agent including a sintered halide-resistant metal impregnated with mixture glass, wherein the adhesive agent is at such a position that is away from a top of the electrode whose rod is held by the electrode holder, by a distance that is out of a range where the mixture glass receives an erosion action of a light-emitting material enclosed in the discharge space at steady lighting.
According to these constructions, such a problem that the light-emitting material enclosed in the discharge space erodes the mixture glass at steady lighting can be prevented. Therefore, cracking damage in the art tube or luminous color change can be prevented, enabling the luminous efficiency to be improved.